S -adenosylmethionine stabilizes cystathionine β-synthase and modulates redox capacity

doi:10.1073/pnas.0509531103

S-adenosylmethionine stabilizes cystathionine β-synthase and modulates redox capacity

*Redox Biology Center and Biochemistry Department, University of Nebraska, Lincoln, NE 68588-0664; and
^†Division of Gastroenterology and Liver Diseases, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033

Edited by Rowena G. Matthews, University of Michigan, Ann Arbor, MI, and approved March 9, 2006 (received for review November 2, 2005)

Abstract

The transsulfuration pathway converts homocysteine to cysteine and represents the metabolic link between antioxidant and methylation metabolism. The first and committing step in this pathway is catalyzed by cystathionine β-synthase (CBS), which is subject to complex regulation, including allosteric activation by the methyl donor, S-adenosylmethionine (AdoMet). In this study, we demonstrate that methionine restriction leads to a >10-fold decrease in CBS protein levels, and pulse proteolysis studies reveal that binding of AdoMet stabilizes the protein against degradation by ≈12 kcal/mol. These observations predict that under pathological conditions where AdoMet levels are diminished, CBS, and therefore glutathione levels, will be reduced. Indeed, we demonstrate this to be the case in a mouse model for spontaneous steatohepatitis in which the gene for the MAT1A isoenzyme encoding AdoMet synthetase has been disrupted, and in human hepatocellular carcinoma, where MAT1A is silenced. Furthermore, diminished CBS levels are associated with reduced cell viability in hepatoma cells challenged with tert-butyl hydroperoxide. This study uncovers a mechanism by which CBS is allosterically activated by AdoMet under normal conditions but is destabilized under pathological conditions, for redirecting the metabolic flux toward methionine conservation. A mechanistic basis for the coordinate changes in redox and methylation metabolism that are a hallmark of several complex diseases is explained by these observations.

Footnotes

^‡To whom correspondence should be addressed. E-mail: rbanerjee1{at}unl.edu
Author contributions: A.P. and R.B. designed research; A.P., Z.B., A.B., and V.V. performed research; S.C.L. contributed new reagents/analytic tools; and A.P. analyzed data.
↵ § The doubling time of SK-Hep1 cells in Met⁺ medium is ≈3 days and could lead to an underestimation of the t _1/2 of CBS under these conditions. In contrast, growth of SK-Hep1 cells stalls in Met⁻Hcy⁺ medium (without a visible loss in cell density) as reported previously for other cancer cell lines (28).
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations:

Abbreviation:

AdoMet,

S-adenosylmethionine.